OMK build: move linker specification of pthread library to separate variable TARGET_L...

OMK build: move linker specification of pthread library to separate variable TARGET_LOADLIBES.

This allows to override libraries list suitable for the Linux
build by other target OS specific list of the libraries.
I.e. the explicit "pthread" library specification is blocker
for QNX/BlackBerry build because POSIX threads are included
in stadartd library. On the other hand BSD networking "socket"
library has to be specified explicitly. Another difference
is missing "byteswap.h" header file (undef HAVE_BYTESWAP_H).

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>